The invention disclosed and claimed herein generally pertains to active power filter devices for reducing distortion in three-phase power distribution and transmission systems. More particularly, the invention pertains to an apparatus and method for controlling an active filter which is coupled to a four-wire, three-phase system to provide compensation for both line harmonics and neutral unbalance.
When a three-phase power transmission or distribution system is coupled to certain kinds of loads, such as rectifier circuits feeding inverter drives and power supplies, substantial harmonic components of current and voltage can be generated in the lines of the distribution equipment. These harmonic components can, in turn, seriously affect other equipment connected to the system, causing, for example, outages of connected computers or light flicker in other equipment. Moreover, in a four-wire three-phase distribution system, a neutral wire is provided which is normally designed to carry only small unbalanced currents. However, if the harmonic component has a third harmonic, or other component which is a multiple of three, the current loading of the neutral wire can increase substantially. Also, higher frequency harmonics in the system tend to cause relatively higher losses due to skin effect. As a result, overheating can occur in a neutral wire, to such extent that a fire hazard is created unless the neutral wire is made much larger than would be required for balanced current operation.
In the past, passive filters, comprising inductors and capacitors, have been used to compensate for the effects of harmonic components in power distribution and transmission systems. More recently, with the advances in power electronic equipment, active power filters have been developed for this purpose because active filters can eliminate harmonic currents more precisely, and are stable over a wide generating range in the distribution network. A typical active power filter comprises a three-phase three-legged pulse width modulated (PWM) inverter and dedicated control. The semiconductor power switches of the three-legged inverter are driven in a PWM mode to generate a set of compensating currents, each of which is coupled to one of the lines of the distribution system. Each compensating current waveform is the same as the load current flowing through the corresponding line, except that the fundamental frequency component of the load current has been separated therefrom. Thus, when a compensating current is coupled to the line, it combines with the corresponding load current to eliminate the undesirable harmonic component in the line current source.
In a four-wire three-phase system unbalanced nonlinear loads can produce substantial neutral wire currents. To eliminate these neutral currents, a fourth phase leg, connected to the neutral wire, must be added to the inverter.
Inverters of this type are exemplified by commonly assigned U.S. Pat. No. 5,280,421, issued to the present inventors on Jan. 18, 1994. FIG. 1 of such patent shows transistor switches T.sub.1 -T.sub.8, which are respectively operated to generate the desired harmonic compensating currents for a four-wire, three-phase distribution system. Various techniques for computing the harmonic compensating currents have been proposed and are based on notch filters, Fourier transforms and instantaneous power calculations. However, it has now been recognized that improved means for controlling the operation of active filters must be developed, to meet new, more restrictive levels of tolerable harmonic distortion and to provide faster response. This is especially true in regard to low voltage distribution systems, (i.e., systems of 600 volts or less) which are used to supply power to computers or other equipment which are highly sensitive to voltage distortion. The present invention achieves this result by providing an active filter control which separates the fundamental frequency component from the compensating currents with more accuracy, precision and higher bandwidth than has generally been available. Also, the invention substantially simplifies such controls.